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If it is a 6 cylinder engine you will probably need to remove the intake manifold to replace the rear valve cover gasket. If that's the case the intake manifold also has two hidden screws in the rear against the firewall as well as an EGR exhaust line that must be removed before the intake manifold will come off. I highly recommend you buy a haynes repair manual for your make/model and a flexible mirror-on-a-stick to use for dis-assembly. The spark plugs can be removed after the valve covers are removed (if the valve cover is leaking you will likely have oil pooled around the plugs anyway).

Replace the valve cover gasket if the valve cover gaskets are
leaking or if they're worn. The procedure is as follows:--1)First of all let the motor cool down if the car was recently driven. Then remove the air
intake boot.The air intake boot ios attached to the engine's throttle body. 2)Carefully disconnect the throttle
cable , hoses and electrical connectors located on the upper intake manifold.3)Then disconnect the hard pipe that is
connected to the EGR (exhaust gas recirculation) valve.4)The EGR valve is located beneath the upper intake manifold.5)Then remove the bolts from both the upper intake manifold and the manifold
support bracket.6)Now the upper intake main fold and
lower intake manifold both have got loose as you removed the nut.7)After that remove the upper intake
manifold from the lower intake manifold. Carefully place clean rags over the
intake runners to prevent anything from falling into the intake system.8)Then you will have to disconnect the
crank case breather valve on the valve cover. 9)Then remove the bolts from the valve
cover and remove the reinforcing straps. 10)Then remove the camshaft belt cover
and the dust shield from the back of the camshaft sprocket.11)Now carefully remove the valve cover.
Properly clean the old gasket material with gasket remover.12)You can install a new gasket in the reverse procedure of removal.

Clean the mating surfaces and install the intake manifold gaskets and seals. Apply sealer to the ends of the of the intake manifold seals.

Position the intake manifold and apply a suitable thread lock compound to the bolt threads. Tighten the retaining bolts to 88 inch lbs. (10 Nm), twice, in the sequence shown in the accompanying figure.

Attach the electrical connector to the temperature sensor and sensor switch.

Install the thermostat housing and thermostat with a new gasket.

Connect the alternator support brace to the intake.

Install the solenoid valve mounting bracket and power steering support brace to the intake manifold.

Connect the heater pipes to the intake and front cover.

Attach the coolant bypass hose to the intake manifold.

Install the power steering pump support bracket and tighten to 37 ft. lbs. (50 Nm).

Install the spark plug wires on both sides.

Install the belt tensioner pulley and tighten to 33 ft. lbs. (45 Nm).

Install the power steering pump.

Connect the vacuum hoses to the canister purge solenoid valve and transaxle module and intake connection.

Attach the electrical connections at the throttle body and both banks of fuel injectors.

Here is a detailed summary on the procedure of how to replace the intake manifold gasket on a 1996 gmc jimmy;
REMOVING THE UPPER INTAKE MANIFOLD(PLENUM)
STEP 1
Disconnect Battery (may not be absolutely necessary, but a good practice to avoid shorting a positive wire to ground).

the thermostat its located in water out hose near to a/c compressor, to change valve cover, front of engine remove the plastic cover with lexus logo using hex 5mm key 2 bolts , unplug 3 spark plug wires ,loose all 10mm bolts on valve cover , to rear engine valve cover remove before the manifold intake and all components , air filter duct , etc.

Carefully place the upper intake manifold onto the lower intake manifold. Ensure that the alignment pins in the upper intake manifold align with the holes in the lower intake manifold.
NOTE: Apply thread lock compound, to the bolt threads before assembly.

when u replace the head gaskets u must also replace any parts gasket that u remove (valve covers, intake manifold etc.) to get the heads off, they sell what is called a "Valve Grind" gasket set that is what u need, it will have all the gaskets in it u need..

do you mean the valve cover ? because I don't think you can have the intake manifold backward.(if it's backward, you cannot connect the upper hose and the fuel lines)
so any way, it sound like you have the valve cover swapped. it won't hurt to drive but it is a problem to fill the engine oil as you stated.

Generally, the valve cover gasket, PCV valve, vacuum lines, exhaust manifold gaskets, and injector seals. These are all things that, universally, should be inspected or replaced at large intervals (60,000 - 70,000) as a matter of preventative maintenance.